G smiley



Aug. 16, 1938.

G; SMILEY ELECTRON IC MUSICAL INSTRUMENT Original Filed Dec. 4, 19:55

6 Sheets-Sheet l Aug. 16, 1938. s. SMILEY ELECTRONIC MUSICAL INSTRUMENTOriginal Filed Dec. 4. 1933 6 Sheets-Sheet 2 MAS TEE 0567114 UE Aug. 16,1938. G. SMILEY 20,331

ELECTRONIC MUSICAL INSTRUMENT 6 Sheets-Sheet 3 Original Filed Dec. 4,19:53

IIIIP Aug. 16, 1938. e. SMILEY ELECTRONIC MUSICAL INSTRUMENT 6Sheets-Sheet 4 Original Filed Dec. 4, 1933 n A a v m W U E w m m e m 5 Er r 4 H W n A V nlin 6 Sheets-Sheet 5 Firs/17v:

W M m G. SMILEY ELECTRONIC MUSICAL INSTRUMENT Original Filed Dec. 4,1933 Aug. 16, 1938.

f Re. 20,831

Aug. 16, 1938. s. SMILEY ELECTRONIC MUSICAL INSTRUMENT Original FiledDec. 4, 1953 6 Sheets-Sheet 6 Reiuued Aug. 16, 1938 UNITED STATESELECTBDNIG MUSICAL INSTRUMENT Gilbert Smiley, Illngham, Mass., assignor,by mesne assignments, to Hammond Instrument y I Compa Chicago, 111., acorporation of Delaware Original No. 2,083,246, dated June 8, 1937,Serial No. 700,836, December 4, 1933. Application for reissue March 28,1938', Serial No. 198,617

23 Claims.

The present invention relates to the production of musical sound byelectrical means and has for an object to provide an oscillatorythermionic vacuum tube circuit which will produce an electric wave formwhich is extremely rich in harmonics, the circuit preferably includingsuitable filter means by which those harmonics desired can be retained.Stated otherwise, the circuit of the present invention is one producingl0 oscillations of a wave form having the harmonics necessary to adaptit to imitate or reproduce sound obtained in the playing of any giveninstrument by inclusion of appropriate filter means acting to suppressor attenuate certain of the harmonics, as well as to create new tonerelationships not necessarily found in existing musical instruments, butwhich may be found desirable.

Another object of the invention is to provide an oscillatory circuitwhich is both simple in construction and reliable in operation, thefrequency of its oscillations being characterized by extreme stabilityand capable oi being determined or computed with ease and accuracy.

Still another object of the invention is to provide an oscillatorycircuit which may be readily adjusted to cause it to oscillate at theproper frequency. i,

A further object of the invention is to provide an oscillatory circuitcapable of having the frequency of its oscillations readily alteredduring operation to predetermined values so that it may be eiiectivelyemployed in the playing of diflerent musical notes. I

A still further object of the invention comprehends the grouping of aplurality of such oscillatory circuits each adjusted to a predeterminedfrequency, whereby various chords may be struck.

pect the provision of. the necessary number of oscillatory circuitsabove mentioned, filtering devices and-switching arrangements andappurtenances for duplicating the eii'ects obtained by -i5 the variouspipes, reeds or other elements of The invention comprehends' in-itsbroad asfor one or more of the note-producing oscillatory circuitswhereby any tendency of a change in the tuning of some .or all of thenoteswill be automatically corrected. 7 The master control also providesa means for shifting the frequencies of a whole series of oscillatorswithout affecting the tuning of the notes with respect to each other.

To these and other ends the invention also includes other arrangementsand combinations of elements, all as set forth in the followingspecification and particularly pointed out in the appended claims.

In the drawings:

Fig. 1 is a diagrammatic illustration of an oscillatory circuitembodying the invention, illustrated in a simple form to facilitate thedescription of the fundamental principle of the invention.

Fig; 2 is a similar view showing another way 7 in which the plateand'grid potentials may be applied to the tube circuit.

Fig. 3 is a modification showing the use of separate transformerwindings in lieu of the tapped mutual inductance of Figs. 1 and 2.

Fig. 4 is a diagrammatic layout of a single tone Fig. 6 is a viewsimilar to Fig. 5, but in addi- Q tion showing the master oscillator andcoupling means through which the control may be effected from oneoscillatory circuit to another.

Fig. 7 is a modification of Fig. 6 showing an intermediate repeater tubeincluded in the coupling between oscillatory circuits.

Fig. 8 shows the possibility oi. associating with each oscillatorynote-producing circuit a means for automatically attenuating theintensity of its note upon key'depression.

Fig. 9 is a modification of Figs. 6 and 7, show- .inga multi-elementiiube substituted 'for the tubes V2 and VI of these figures.

Fig. 10 is a view showing the possibility of associating with eachoscillatory note-producing circuit a means whereby the intensity of itsnote may be varied at will by the distance its controlling key isdepressed.

Figs. 11 to 14 are curves showing corresponding changes occurring. withtime in the grid-cathode voltage, plate current, plate-cathode voltageand grid current of the tube of the oscillatory circuit of theinvention, respectively.

Fig. 15 shows two consecutive cycles of the output wave as derived fromthe grid-cathode Fig. 17 illustrates two sinusoidal wave forms 0 Idiiferent, but 'harmonically related frequencies.

Pig. lbillustrates the grid-cathode voltage wave of the tube of theoscillatory circuit of the inventionandthetrigger actionoia slmilarwavesuperposcd thereon by a master or control oscillator.

Figs. 19 and 20 show embodiments of the invention wherein the outputwave is derived from the grid-cathode branch of the oscillatory circuit.

Similar reference characters in the several figures indicate similarparts.

In carrying out the invention the fundamental object of which is toprovide a new apparatus for the production of musical sounds, there isprovided a vacuum tube oscillatorwhich will produce audible frequencyoscillations the wave form of which lends itself to musical use. Inother words an oscillatory circuit constructed in accordance with theinvention will produce a wave which possesses at any of the frequenciesto which the circuit may be tuned the harmonics required to give thedesired musical color or timbre. so that thecircuit may be adapted tomatch or reproduce the notes of any given instrument, as well as togenerate its own variations of tone, by proper inclusion oi' adiustablefiltering elements. The circuit which is provided includes the furtheradvantage that it gives an immediate full response upon the closing of akey without time lag incident to building up of the energy required forits operation-and produces a sustained note of the predeterminedfrequency during the period of key depression. These factors areconsidered important characteristicszof the invention and enable theproduction of musical sounds with great fidelity. Furthermore, it ispossible to duplicate the effects produced by a complicated instrument.such as an organ. by means of apparatus capable of assembly in acomparatively small space.

The oscillatory circuit of the present invention establishesaudio-frequency oscillations by the time constant characteristic of acondenser discharging through a resistor. Generally stated, such circuitemploys a thermionic vacuum tube in association with a potentialfeed-back means closely coupling the plate-cathode circuit through acondenser to thegrid, the condenser-being in electrical connectionthrough the feed-back means with the cathode and therefore in aclosedcircuit relation throughthe cathode with a resistor included inthe grid-cathode circuit. The

condenser alternately receives a charge and dis ing properly poled forthis purpose. Proper choice should obviously also be made of thesupply-potentialstthegrid. Itwillbeapparentfromthe following descriptionthat no precise constructionof the interiorelements of the vacuum tubeis essential and that, although the conventional type-,ogthrceelementthermionic deviceisillusao,ssi

trated in the various figures. other types of multielement tubes asdistinguished from a triode tube may be used with equally satisfactoryresults.

The oscillatory circuit illustrated in Fig. 1 comprises a thermionictube if having a cathode K, a gridGandananodeP. Asourceof potential,such as a battery b. has its negative terminal connected to the cathodeK and its adjustable its operating point, the grid is biased beyond thecutoif point of the tube and no substantial current flows in the platecircuit.

The elements R, C, M and the two sources of potential are each variableat will. By their adiustment the fundamental oscillatory frequency ofthe circuit can be established in the first instance and maysubsequently be changed as desired. The chief methods of controlling thefrequency generated by the circuit are by adjustment of thevalues ofresistance R. and condenser C and by adjustment of the supply-P tentiallie. By using fixed values of R. and C of approximately the proper.relative values. final tuning may be charge has been accumulated in thecondenser C that the grid G of the tube V is suiilciently negative toprevent passage of plate current. Obviously the condenser will notretain this charge as there isa path through the resistor R, theinstrumentalities producing the potentials Be and Eb and the winding L0,around which it can discharge at a rate determined by the raistance oftlus circuit.

ThecondenserCdischar-gesaccordingtothe mathematical relationship thatv,=v,. where V. =the terminal voltage of the condenser at any instant; V=the terminal voltage of the condenser at time sero, I c =the naturallogarithmic base, t =the time in seconds from time zero,

and, li'andc'aretheresistanceandcapacity respectively of thecondenser-discharge circuit.

Deslgnatingtheperiodofdischargeofthecondenser C during the operation ofthe circuit by thesymbol T,theratio Vr/Vn attheendoi'such periodis e?and,forgivenvahiesoflt' andO' andregardleof the initial voltage value Vo, is a factor of the period T and of the period T alone.

As time progresses, therefore, the condenser C will continue todischarge through the resistor R and the effect of this discharge willbe to render the grid-cathode voltage (Fig. 11) progressively lessnegative until, depending upon the parameters of the tube V, the time Twill be reached when the grid-cathode voltage is such that plate current(Fig, 12) can begin to pass, provided that the grid supply-potential E0is insufiiciently negative of itself to produce cutoff conditions. Assoon as plate current starts, and depending upon its rate of change, areactive voltage is set up in the winding Lp of inductance M, the latterwind= ing opposing the change in the status quo. Through the medium ofmutual inductance M, however, induced voltage is transferred to thewinding La, which causes the grid-cathode voltage to increase in apositive direction, which, in turn, through the action of the triode,tends still further to increase the plate current. Thus aself-accelerating or regenerative action is created, wherein increasingpositiveness of the grid sets up increasing plate current, which stillfurther increases the grid potential in a positive direction. As theplate current increases, however, the efiective plate-cathode voltage(Fig. 13) decreases owing to the opposing action of the reactive voltageset up by the rise of plate current through the winding Lp. Similarly,the increasing positiveness of the grid causes the gridcathode voltageeventually to pass through zero from the negative to the positiveregion, and the grid-cathode circuit becomes conducting, at which pointthe grid starts to permit current to pass from the grid to the cathode(Fig. 14), which current causes a charge to accumulate in the condenserC. Shortly a limitation is reached corresponding to which the condenserC no longer accumulates a charge and the grid, for this and otherreasons, ceases to become further positive. At this moment plate currenthas risen to a maximum and then a reverse regenerative action takesplace. Assoon as the rate of change of plate current ceases to increase,both the induced voltage in the winding La and the grid potentialdepending upon it start to decrease. This decrease or return towardnegativeness of the grid potential causes the'plate current to decreaseand the grid-plate regenerative action again takes place, though now inthe opposite direction. As soon as the grid-cathode voltage passes outof the conducting region, the charge accumulated in the condenser C bythe passage of' grid current is locked in, as it were, and can onlyescape by the way of the path offered by the resistance R. The platecurrent rapidly falls to substantially zero and the grid is now negativebeyond the'cutoff point because of the charge present in the condenserC. This charge includes the normal charge present from the combinedpotentials Eb and Ec, and is caused by a transfer of energy from theplate circuit to thegrid circuit of the tube, a portion of which energygoes to make up the condenser charge, which may be represented I by thesymbol Q. From the time the gridcathode voltage passes out ofthe'conducting region to the time that the plate current ceases. such asmall interval elapses that the discharge of condenser C throughresistor R during this interval is, "for .all practical purposes,negligible. However, such discharge does take place, and so cannot beoverlooked; Thus, when the plate current cycle of operation iscompleted, the gridcathode voltage is (2/0 minus a small amount, where Qrepresents the maximum charge (over and above that charge induced by thepotentials Eb and E0), which was acquired during the action justdescribed. From this point on resistor R and condenser C take charge ofthe operation of the circuit.

Referring to Fig. 11, the condenser C discharges from the voltage V e,equal in value to Q/C, to the voltage V causing cutoff potential at thegrid G, at which instant the cycle of operations is repeated. Dependingupon the constants of the circuit to some extent, the charge Q and thevoltage V'o depending upon it are determined by the magnitude of theplate supplypotential Eb. Likewise the cutoff voltage V is determined byEb through the medium of the amplification constant of the tube.Supposing E0 to be zero, it is obvious that, other things remainingconstant, if Eb be increased or de-' creased by some factor, Vgo and vglwill be similarly changed by the same factor. And, since the ratio n l/0 R!1;:!

changing both terms of that ratio by the same factor will leave theexponential term unchanged.

' Thus, for given values of R and C time indicated by T must remainfixed so that, since the cycle of plate current passage is over in aminute fraction of the time required for the entire cycle, the frequencyof the established oscillations is determined by the period of condenserdischarge almost to the exclusion of all other factors.

Under the circumstances when E0 is not zero, it will be found that Ecmust be changed in proportion to Eb if the same frequency of establishedoscillations is to be maintained. Variation in Elc acts to change thepoint of time on the discharge curve at which cutoff potential isreached. The cutofi' potential for zero grid supply-potential, or thecondenser discharge voltage Vn, may be represented by the ratio Eb/u,where u is a constant of the tube. If now the grid supply-potential ismade negative (by connecting the negative terminal of theinstrumentality c to the resistor R), the condenser 0 must discharge toa voltage V equal to Eb/u minus Ec, rather than simply Eb/u as is thecase when Ec=0, the voltage Va being therefore smaller than the voltagev.1. Referring to Fig. 16, the condenser discharge period T when Ec=0 isindicated by TI. Since Va is smaller than V substituting Va in theexponential equation 'must-of-necessity increase the period T toadefinite value T2, all other factors being constant. Similarly, if E0 ismade positive, the period T will decrease to a definite value smallerthan Tl, as the ultimate condenser discharge voltage V will be largethan v...

It will be apparent from the foregoing discussion thatto keep thefrequency of the established oscillations constant, it is necessary tochange E0 in proportion to Eb. but that, regardless of the value of E0,such frequency is substantially exclusively determined by the period ofcondenser discharge. The periods of time between condenser dischargesare indeed extremely short and are purposely exaggerated in Figs. 11 to16 for clarity of illustration, it being aim apparent, that the waveform of the oscillations established in either grid-cathode orplate-cathode circuit (see Figs. 15 and 15a) is extremely rich inharmonics.

So far the discussion has concerned itself with ill The operation of thecircuits of Figs.l and 2 may be considered as typical of otherarrangements which may be utilized to produce audiblefrequencyoscillations of a wave form suitable for musical instrument purposes,wherein the coupling between the plate-cathode circuit and the grid issuiilciently close to permit the accumulation of a sumcient charge in agrid condenser to produce grid potentials in excas of cutofl potentialand wherein the condenser in discharging through a resistance determinesthe time period or frequency of the established cscillationa, Theoscillatory circuit, for example,

maytaketheformshowninrlg.3.wherea transformer M having its windingsclosely coupled to one another is substituted for the auto-transformer Mof Fig. 1. The action of the circuit of Fig. 3 is practically identicalto that g of Figs. 1 and 2, but here the supply-potential lb isseparated from the condenser-discharge circuit. Similarly, any existingmeans of securing feed-back may be employed if suitableprovisionismadetoassurethatthechargeaccumulated by the grid condenser issui'licient to carry the grid potential beyond the cutoff point of theparticular vacuum tube employed, so that the discharge characteristic ofthe condenser is the major factor in frequency determination.

The type of oscillator described is especially adapted for the purposeintended. as it maintains a substantially constant ratio of the limitvoltages between which condenser discharge takes place. regardless of.wide fluctuations in its supply potentials Eb and He, provided theinstrumentalities producing these potentials are'numberofnotesdesiredarebridgedacrosstheswitchesLLandIbeingrespectivelyconnected between the outer terminals ofthe resistances RI, R2, and! and the sliders la. is and is of thepotentiometers pl, p2 and p8. 'Tuning foreachoftheseparatenoteswithintherangeoftheinstrumentmaybeeifectedbymerelyadjusting the sliders la, 2a, In.which determine the individual grid-NW P tentials Eel, Be! and Eat.Obviously, the frequency of the note producedhydepressingkey Iisdeterminedby the discharge of the condenser C through the resistanceRI, the frequency of the note produced by depressing key 2 is determinedby the discharge of the condenser C through the resistances RI and RI,and the frequency of the note produced byde key 3 is determined bythedischarge of thejcondenser C through the RI, R2 and R3.

In the apparatus shown in Fig. 4 there is included in the circuit ofcathode K an adjustable tuning redstor at by means of which the chargeaccumulated in the condenser C may be varied to alter the pitch of theinstrument as a whole. 'Ihatiathcresistancelttisusedtotuneali notessimultaneously and thereforeprovides a convenient means of tuning theinstrument with others or changing its tuningto conform with recognisedpitch, such as international, orchestra or concert. In addition to theforegoing there is included invseries with the several key resistancesanother resistance RI having its outer -potential Eb during eggmpply-potentialattheplaterandto the oscillatory plate-cathode circuit offrom the instrmnentality 1:, therebyventingthecreationofopposingvoltagesthat IthmmfliemltplltWiVedthe I ingcondenser C6 is introduced to prevent the plate ao,ssi

oscillator is amplified by a thermionic amplifier i of suitableconstruction and thereafter reproduced in the desired volume by means ofa suitable loud speaker I. This wave is preferably fed to the amplifierthrough an adlustabie filter to permit the tone of the instrument to becontrolled by cutting out those harmonics or component frequencies whichit may be desired to suppress. Indicative of such a filter there isshown in the output circuit, comprising the wire 0, a resistance R! withwhich two condensers C8 and C4 of difierent capacities are associated.These condensers are connected at one side to a conductor I leading tothe resistance RI and at the other side to terminals II and II, withwhich cooperates a switch arm I! connected (through the resistance Rt)to the cathode K. Between the resistance R! and the plate P ablocksupply-potential Eb from acting through the output net work, whichcondenser C6 is of sufiicient capacity to permit the free passage of thealternating potentials developed. .The resistance R5 functions as afilter resistor and isolating resistor and is of such a value that, evenif a short circuit occurred at one of the contacts of the filter switchl2, no appreciable change in the frequency of the establishedoscillations would occur.

Also arranged in the wire 0 isa second resistance R8 and a potentiometerP4,- the latter being connected to the amplifier 0. The resistance R6limits the maximum volume of sound obtainable, while the potentiometerP4 acts as a volume control, its slider being adjustable from zero tothe maximum voltage obtainable.

In practice the plate supply-potential Eb and the potential Eel, as wellas the potential for the cathode-heating element H (where a heater typeof tube is used) are derived from the power supply already available inthe audio-frequency amplifier 8 used with the oscillator. The amplifiermay be of conventional design. with the addition, if desired, of otherfiltering means for the further control of tone quality. By using thepower supply circuits of a commercial amplifier, as a source of supplypotentials, the amplifier, in turn, being operated from the power means,and by supplying the potential required across the potentiometers Pl, P2and P3 either from the power mains with a separate power supply deviceor from the power supply circuits of the amplifier, the varioussupply-potentials will in a properly designed oscillatory circuit beproportional to each other, or nearly so, at all times. That is, theoperation of the device will be substantially stable with relation tosupply fluctuations.

The principles thus far disclosed in their relation to a single toneinstrument are equally applicable to an instrument capable of producingharmonies. Fundamentally there would be provided for such a purpose aplurality of oscillators such as that of Fig. 1, each tuned by means ofthe proper choice of the resistance R, capacity C and biasing potentialEc to a predetermined fundamental frequency. Appropriate switchingmeans, such as keys, would be provided, each oscillator being soarranged that, when its key is depressed, a proper proportion of itsoutput is caused to be conveyed to the input of an amplifier feedinginto a loud speaker. Such anapparatus when properly assembled and tunedcan be made to play chords and in its essence is the basis of what maybe termed an electric organ. By the addition of filter devicescoordinated with stop-type controls, any desired change of timbre ortone shading can be accomplished, so that the apparatus will duplicatevarious instruments, or may be used for the purpose of originating itsown discrete tonal values, as distinguished from existing musicalinstruments. Likewise, all notes can be made subiect to either manual orautomatic vibrato, and to volume changes accomplished by either knee ormanually operated controls, such as are now used in organs for producingsuch efi'ects.

While it is possible to evolve circuits by which one oscillator can becontrolled from a variety of keys, so chosen that no two keyscontrolling the same oscillator would need to be simultaneouslydepressed in the rendering of formal music, it is considered best toassign an oscillator for each desired note in order that any desiredcombination of keys can be simultaneously depressed to produce therelated chord sounding in each case.

Fig. 5 illustrates a circuit arrangement which may be used inconstructing such an apparatus, two oscillators being shown for thepurpose of simplicity. The two oscillators are identical, ex-

cept for the choice of the values which should be used for theresistances Ri and R2, condensers Cl and C2 and grid supply-potentials,Eci and E02 in order that the two notes desired may be produced. In thisarrangements parallel feed may be used to connect the plates of theseveral tubes to the source b, resistors Rfl and R12 of high value beingemployed through which the potential Eb supplied by said source isdirectly communicated to the plates PI and P2 of the two tubes VI and V2illustrated. The potentiometers pi and p2, of course, serve for finaltuning after the latter has been approximated by choice of theresistances RI and R2 and condensers Cl and C2. The condensers Chi andCM, respectively connected in the output circuits ofthe tubes VI and V2,isolate the steady potential Eb from these circuits and prevent itstransfer across the plate-circuit windings of the inductances MI and M2,but are so designed as to freely pass the alternating potentialsdeveloped. Besides the condensers Chi and Cb! the output circuits ofthetubes bl and b2 respectively comprise the resistances Rkl and R102 andthe keys II and :2 associated therewith, the potentiometer or volumecontrol p4 being common to both circuits with both keys depressed. Theresistances RJcl and RM are so chosen that the voltage appearing acrossthe impedance of p4 when one of the noteproducing oscillators is beingsounded through its associated key, say the circuit of tube VI, will notbe coupled back to any appreciable extent through this impedance intothe other note-producing oscillator, the oscillator comprising the tubeV2 under the assumed condition. Likewise the impedances of theinstrumentalities supplying the operating potentials at the plates andgrids of the tubes should be kept low in order to prevent undesirablecoupling effects.

As is shown in Fig. 5, a capacity C! is connected across thepotentiometer pl as indicative of a simple means which may be employed.By making this capacity variable between the desired limits variousamounts of the higher harmonics may be retained or suppressed at will.

The arrangement of Fig. 5 enables excellent stability of tuning to beobtained. However, there are variable factors such as the effects ofhumidity on leakage paths, the change in tube 75 ticnoi any other. s

Asanexamplaletusconsiderthe relationship and the note next above it onthe tempered scale. This relationship very nearly meets the conditionthat, for every four there will be very othirdcycleofthenotemthetwonotcs'maybe itiallysounded together. In m. areshown two sinusoidal waves il and II having the above-mentionedfrequency refrequency higher fre- ,the two wavespassthroughthe Now'l'ig.18 shows graphically controlling oscillator. from which oscillations aresuppliedtothecontrolledoscillatorinamanner to be hereinafter described,is assumed to operate at the frequency corresponding to the higherpitchnotecofthetemperedscale. 'lheoscil-- similar in wave form to theoscillations pro- 5 oscillations the controlled oscillator controlledoscillator being to produce certain irregularities lid in itscondenser-discharge portions, corresponding to which the grid approachescutofl potential in a positive direction. These denser discharge in theoscillator of the present invention. there is a considerable period whenthe grid-cathode voltage is very close to the onto! 1 level, but has notquite reached that levei, because of which impulses of comparativelysmall magnitude need be supplied to the grid duringsuchperiodtocauseachangeinthepointof time at which plate current willagain be initiated. Therefore. should the controlled oscillator tendtoosciliateatanyotherthanthedesiredfre- Practioalortlnpend,etc.,whioh,ov eraperiodof tervalsalreadytabulated.itwillbepossible,be-

thelpotentionieterspiand ofallnotescfthetemperedscalatoexprusany t to-beeasily retuned given note of said scale in a whole number fraccfthetubes. whichisoontothrowtheapparatusout-of ,causeofthealmosttruenatural-relationships apparatus of this inventionowtotherelativelylowemlssiondemandsupon betweenthenoteg them.Ohangesintuningmust inevitably occur.

thoughusuallytheyvareofsmallmagnimtudeandforthis'reasonitisdeemedhighlydecompietecyclesot-thecnotesirable to am for automatically nearly'threecompletecyciesotthelowernoteG. maintainingthe fll lofeachnoterelativeto'lhusateachi'ourthcycleoi'thenotecandeach all theothsrs.

Preliminarytoesflaininghowthis'automaticconsiderediobeinphasaprovidedtheyareincontroiiseifectedit'istobeunderstoodthattheI invention, contemplates theme of a master oscillator. This may beone-of the note-producing oscillatory circuits, or aseparate oscillatorycirlationship of four to three, and it will be noted cuitpreferablyoneoseillating atafrequency thatatthethirdcycleofthelowercorresponding to-one or the higher frequencies wave II and the fourthcyclecf'the tothaotheroscillatorainsuchamannerthatany same value(aerolin the same direction (from controlled oscillators normallycausing it to lack the grid-cathode voltage wave Ila of an oscillator ginhannony is checked by controlling impulses constructed and controlledinaccordance with 25 from the output oi the masterosciilator or from theinvention. This oscillator is assumed to opthe outputcf a controlledoscillator. whereby all erate at ,the frequency corresponding to thelower controlled osdllators are caused tooscillate with pitch note G ofthe tempered scale, while the m i'emflai'ityinstepwiththemasteroscil';htot. I

The practical or tempered (chromatic) musical scale as employed in akey-controlled instru-i mentisdivided into twelve intervalstothe octave.

Iach octave interval is a two to one frequency lations produced by thecontrolling oscillator are oessive notes:of. an octave are all equal toone .duced by the controlled oscillator andunder the terval, no harmonicrelation-between by the ratio four to threerthe eflect of the con- 49any two notesof a key-controlled instrument. trollingoscillator on thegrid-cathode wave ofthe In other words, the tempered scale does notpermit the PM of those frequencies whichnaingbeenadoptedforpracticalreasonsduetothe have to be provided as aresult of the frequent trolling wave is suitably attenuated prior tosuthey are neverthelem very close to being so. as shown by the followingtabulation: fnteroalsorirequcncy rdtiosmtemsofflbelow oftbekeyboard.'lhemasteroscillatoristiedin quencywaveli decadence in the operaticnofany one of the plus to minus).

3 ratio and the twelve intervals between the sucture demands intherendition of music, it havextremely large number of keys which wouldirregularities Ila are only slight because the cone mm quency, such animpulse supplied at the proper instant to correct its grid potential tothe cutoif value will cause it to be triggered oil into its on H truecycle of operation. 7 1.2m: Assuming. for example. that the controlledos- 3;} clliator producing the wave Ila illustrated in m. 1.0110 18 hasa tendency to run a slightly lengthened {5' period, then on eachsuccessive cycle this effect would normally become cumulative and 1 W mthe mud overshooting would occur as indicated by the betzree t lmathematically derived intervals dotted line! at he right in Fig-However. on the third cycle the impulse from the controlling variationsbeing-limited to a few per cent, so oscillator corrects suchovershooting and startsviously. the impulse from the controllingoscilscale can therefore be stated to be naturally relator must be smallenough so that it thedesiredpointoftime. The eVn Dsdmsl vabam and theactual musical intervals is relatively close.

slight as to make it diiiicult to detectby the the controlled oscillatoroil! in phase again. Obtrained musical ear. The notes of the temperedlated for all practical purposes. If, now, the retrigger the controlledosciliator at any 'ldmainingsemitoneintervalsbeaddedtotheinsince thisfourth impulse occurs at an instant when the grid potential of thecontrolled tube is sufficiently close to the cutoff level to permit thetrigger action to take place. From these considerations it is obviousthat,'for maximum control. the controlling impulse must be as large aspossible in order to trigger over the widest possible discrepancy, butnot large enough to give rise to a triggering action before the propercycle.

From the foregoing discussion it is apparent that control may beeffected between any two oscillators the frequencies of which aremusically related to one another. Consequently, with all oscillatorstuned to produce the various notes of the conventional keyboard, theycan all, in eflect, be tied together, either directly or through themedium of intervening circuits, until all are made subject to thecontrol of a master oscillator. The latter may be any note-producingoscillator, such as the one producing the note middle C. It is possibleto use one representing the highest pitch note on the keyboard, or aseparate oscillator of still higher frequency may be employed, which isnot used for sounding notes. In practice particular care is taken torender the master oscillator exceptionally stable. Some care should alsobe taken to insure that no controlled oscillator becomes the controller,which may be avoided, by proper coupling between oscillators and properselection of circuit constants. The controlling effect is sufficientlymarked to offset the normal detuning incident to tube aging withinoperating life limits and to small changes occurring in the values ofthe resistances and capacities used. Also, if the frequency to which themaster oscillator is tuned be changed we small amount, the wholekeyboard is shifted by. that same amount, which provides a means ofsecuring tuning to pitch. As another advantage, independent tuning ofthe various note-producing oscillators is greatly facilitated. That is,if any one of these oscillators is out of tune, such an oscillator willgive indication of this fact by a slipping or beating eil'ect.wh ichonly needs to be eliminated by adjustment of its circuit constants, toindicate its proper tuning. there being no possibility of doubt when atrue tie-in is achieved. Still another advantage resides in the factthat tremolo or vibrato eflects may be introduced into all of thecontrolled oscillators from a single point, namely, the masteroscillator. Any suitable means for periodically varying the frequency ofthe master oscillator at a vibrato rate may be employed, as most simplyillustrated in the patent to Langer No. 1,832,402, granted November 1'7,1931.

The frequency relationships between controlled and controllingoscillators must be chosen with a thought as to the simplest fractions,as the smaller the numerator and denominator of the fraction are, thewider the region of natural period change over which the trigger actionmay take place and the more readily control may be effected. A carefulanalysis of the tempered scale shows that tie-in sequences betweencertain notes may be so selected as to give a maximum of control withcorrect musical tuning, as the frequency relationships practically takethe form of simple fractions, as already explained. In practice, asingle octave may be completely tied in, directly or through a sequence.to the master note. which may well be the upper C of that octave, andthen other octaves may be tied in progressively below the master octave,thus completing a tie-in of the entire keyboard oi. the instrument. Inthis manner I have found that, by selectively grouping the variousnote-producing oscillators so as to cause the oscillators of eachselected group to feed through a common output and by connectingappropriate filters one in each of the common outputs of these selectedgroups, any desired predetermined wave form for the keyboard as a wholemay be attained.

Fig. 6 illustrates a means which has provedeffective in practice forsecuring a tie-in of a plurality of note-producing oscillators with themaster oscillator. The arrangement shown is for a multi-tone instrumentand. except for the introduction of the master oscillator and thecoupling elements by means of which the tie-in is effected,

is the same as that of Fig. 5. The master oscil-' lator is shown forsimplicity as a rectangle II, it being understood that its oscillationsareestablished like those of ,the note-producing oscillators inaccordance with the fundamental circuit of the invention. The masteroscillator is coupled to the oscillator comprising the tube VI by meansof a condenser C0 and resistances Re and Rd and the latter oscillator iscoupled to the oscillator comprising the tube V2 by means ofa condenserCal and resistances Rel and Rdl, the resistances Re and Rci beingrespectively connected in the grid-cathode circuits of the two tubes Viand V2.

Thus the oscillator comprising the tube Vi is directly controlled by themaster oscillator, while the oscillator comprising the tube V2 isindirectly controlled by the master oscillator through the intermediaryof the controlled oscillator comprising the tube Vi. In this manner itis possible to tie-in any number of note-producing oscillators with themaster oscillator, the condenser Ca2gand resistances R412 being shownconnected to the output. of the tube V2 for the purpose of illustratingthis possibility. For a complete understanding of the apparatus shown,the function of the condenser Cal and resistances Rcl and Rdl, providingthe coupling between the oscillators comprising the tubes Vi and V2,only need be described, it being understood that each of the couplingsprovided between oscillators operates in identical manner. The condenserCgl acts as a blocking condenser by means of which the grid supply-Ptential of the tube V2 is isolated from the oscillatory plate-cathodecircuits of the tube VI, but is so designed as to freely pass thecontrolling impulses supplied from the tube VI. The resistances Rel andRdl function as an ordinary voltage-divider, their values being sochosen that they will attenuate the controlling impulses introduced intothe grid-cathode circuit of the tube V2 to an extent determined by theconsiderations for maximum trigger action for the harmonic relationshippeculiar to the respective notes produced by the oscillators comprisingthe two tubes VI and V2. The resistances Rcl and Rdl also serve toattenuate impulses from the controlled tube V2 to the controlling tubeVI, thus eliminating to some extent the possibility of reverse control.

Another means by which assurance can be had that the controlledoscillators are always govemed by the master oscillator "is illustratedin Fig. 7,-which shows the use of an isolating thermionic repeater tubeV! inserted between the controlling tube VI and the controlled tube V2;In

by such filter networks improvementin the control may be obtained.

In an instrument oi the character describedl'brinstancesomeorallofthekeys,insteadofmerelyclosingswitcheamaybecausedtovary the intensity oi'thenotesasproduced, ameans tothis end being illustratedinFll. 10. Referringasanoutputcouplingsoastosupplyinvarying amount the outgoing oscillationsfrom said oscillstortotheinputofarepeatertubevll, the resistance Rkibeing in thisinstance connectedin theoutputcircuitoi'thetubevll. Thesliderlluoftheresistanceisatallflmesineleca tried connecflon with thegrid Gll of the repeater tube, but is normally in contact with the seropotential end 'of the resistance 2i so that oscillations from the tubeVi are suppliedthrough the repeater tube Vll only upon depreuion of 4the key ll, the note during its production being obviously capable ofhaving its intendtr resulatedatwillbythedistancethekeyllisdepressed. Asshown, a blochng condenser fl is connectedwiththe'resistanceIiintheoutput circuit of the tube Vi. another'blocklng condenser llbeing connected with the ruistance mi in the output circuit of the tubeVll.

As a ,i'urther elaboration of the invention, some or all of the notesmay be automatically attengguatedinintensityduringtheperlodofkeydepression and thus produce an effectsimilar to that of a plucked string, this concept being illustrated inPig. 8. Here the osdllator comprising ,the tube Vi directly supp ies itsoscillations Q through a biocking'condenser ll to a repeater tubeVilhavingacondenserCil anda resistanceRilconnectedinparallelwithoneanothersnd in series with a resistorRii,'in its grid-cathode circuit,thenoteproducedbysaidoscillatorbeing Qcontrolled byakeyli adapteduponitsdepressiontocausethebiasatthegridGiloftherepeater'tube to beprogressively-increased by' dischargeofthecondenaercilthroughtherwstanceRil. The key li is operatively associated "with the contact 8i movabletherewith and adaptedtonormallyongaseaflaedoontactsl,stillanothercontactslbeingprovidedtoestablishconnectionwiththecontactBiduringkey depression. Do satisfy the requirements of the II apparatusunder discussion, the betweenthecontactssiandBlshouldbeeifeotedmomentarily and only during the downwardstrokeofthekeyl.i,torwhichpurposetheconpermitting the contact Bl to wipepast the end portion of the strip during its downward movement and towipe pasttheinsulation idurlngits 1o upward movement. The condenser Cilis adapted to be normally short-circuii'ed by the contacts 8i and 82,the movable contact 8i being con-, nected to the grid side 01 thecondenser Cil and the fixed contact 82 to the cathodeside. For 15reasons which will shortly appear, two sources of opposite polarity areoperatively auociated with the grid-cathode circuit or the tube Vil,thes'esources beingindicatedintheembodiment illustrated by batteries cland cl. The battery so cl is permanently connected in the grid-cathodecircuit so that potential therefrom is at all times supplied to the gridGil, while the battery c2 is soconnectedthatitsvoltageisimpressedacross'the condenser Cll and potential therefrom-is g5 supplied to the'grid Gilduring the short infervaloftimethatthe contactsSl andslarecaused toengage with one another, the flexible contact Sl being-connected throughthe battery c! to the cathode side of the condenser Oil. The batso .atthe grid Gil. the battery cl having a voltage value such as to increasethe potential at the grid Gil to an operating value by its opposition tothe battery ci when rendered efl'ective by en- Basement of the contactsSi and Sl. It is thus 40 apparent that oscillations from the tube Vi aresupplied through the repeater tube Vil only upon key depression. Thenote is initiated at the instant connection is made between the contactsBi and SI and, because of the momentary char- 5 acter of thisconnection, the condenser Cil is almost instantly charged by the batterycl. Thereafter this charge is progressively dissipated through theresistance Ril, the effect of which is to progressively increase thebias at the'grid 5o Gil. This changing bias alters the amplification ofthe repeater tube, which amplincationisgraduallydiminishedwiththeresultthatthenoteil correspondingly mduallyattenuated in intensity during its production. The action is comparableto that obtained in a piano wherein the hammer corresponding to the key,depressed strikes the string and bounds clear until the key is againdepressed. Moreover, when the key Ii is released, the contact Bi makespermanent conso nection with the contact 82, thereby short-circuitingthe condenser cll and abruptly terminating the sounding o! the note.This embodiment of the invention thus duplicates the elect obtainedbythestrikingota pianokey sndsubsedo quent release thereof, or by theplucking of a string and subsequent finger contact thereof. As shown,the resistance Rki and a blocking condenser ll are connected in theoutput circuit of the tube Vil. 7o

Inl'll.9thereisshownamodifledcircuit wherein a multi-eiement tube V20performs the separate functions of the tubes VI and W of Fig. 7. Thesingle plate Pll performs both thefunctionoftheelementPloithetubeVlandthen function of the element P3 ofthe tube VI, by

operating in conjunction with the two grids 6V2 and CW, which performthe functions of the grids GI and GI oi the tubes V2 and Vi,respectively. The grids CW! and GVI are interposed in the electronicpath between the plate PII and the cathode K", and one of these grids issurrounded by a perforated shield which receives a supply-potential froman external source cs. The controlling impulses are here atten by theresistances It and ti, which serve together with the blocking condenserCal to couple the controlling tube Vi to the grid 6V3.

In the embodiments which have been described, the output wave is derivedfrom the plate-cathode side of the oscillator and is thereforerepresented by the curve shown in Fig. a. It is also possible, however,to derive the output wave from the grid-cathode circuit, in which caseit will then It take the form shown in Fig. 15. It is understood,

of course, that either form is rich in harmonics and adaptable tomusical purposes.

In Figs. 19 and 20, there are shown embodiments of the invention whereinthe output wave is derived from the grid-cathode circuit. Thus, ineither of these figures, the simple oscillatory circult of Fig. l isshown supplying oscillations from its grid-cathode circuit to athermionic repeater tube V", which merely serves to amplify andcommunicate such oscillations to the output terminals o. The latter, asis obvious from previous description, may be connected throughadditional amplifying means, filter means or other accessoriesdescribed, to the loud speaker. In Fig. 19 the intensity of the noteproduced is varied by a potentiometer p30, one end and the slider .ofwhich are connected to the terminals 0. In Fig. 20, the grid G30 of therepeater tube is connected to a slider cooperating with the resistance Rof the oscillatory circuit, the adjustment of the slider eflecting avariation in the intensity of the note. Except for these differences,the two arrangemenis shown in Figs. 19 and 20 are similar in operation.In either case, the grid and plate supply-potentials ECR and EBR are soselected that, during operation, the potential at the grid Gill does notpass beyond cutofl or into the positive region. In this way, the voltagewave across the grid-cathode branch of the oscillatory circuit may beextracted without" substantially aflecting the operation of the lattercircuit, the repeater tube circuit serving to effectively isolate theoscillatory circuit from the accessories in electrical connection withthe output terminals 0.

What is claimed is:

1. In a device for the production of musical sound, an oscillatorcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a plate-cathode circuit and a grid-cathode circuit, aresistance connected in said grid-cathode circuit, feed-back meanshaving two closely coupled portions one of which is connected in saidplate-cathode circuit, a condenser connected at one side to said gridand at the other side to the other of said portions, said condenserbeing in electrical connection through said otherportion with saidcathode, and potential-supplying means for operating said tube, saidfeed-back means being poled to cause said condenser to periodicallyrapidly accumulate a charge such as to bias said grid beyond cutoflwhereby its period of discharge through said resistance substantiallydetermines the frequency of the oscillations.

2. In a device for the production of. musical ll sound. an oscillatorcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a plate-cathode circuit and a grid-cathode circuit, aresistance connected in said grid-cathode termines the frequency of theoscillations.

3. In a device for the production of musical sound, an oscillatorcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a

grid-cathode circuit and a plate-cathode circuit, a resistance connectedin-said grid-cathode circuit, an inductance having a portion connectedin said plate-cathode circuit, a condenser connected at one side to saidgrid and at the other side to the other portion of said inductance, saidcondenser being in electrical connection through said other portion withsaid cathode, and potential-supplying means for operating said tube,said inductance being poled to cause said condenser to periodicallyrapidly accumulate a charge such as to bias said grid beyond cutoi!whereby its period of discharge through said resistance substantiailydetermines the frequency of the oscillations.

4. In a device for the production of musical sound, an oscillatorcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a

plate-cathode circuit and a grid-cathode circuit, a resistance connectedin said grid-cathode circuit, feed-back means having two closely coupledportions one of which is connected in said platecathode circuit.=acondenser connected at one side to said grid and at the other side tothe other of said portions, said condenser being in electricalconnection through said other portion with said cathode, and means forsupplying operating potentials from a single source to said plate andgrid, said feed-back means being poled to cause said condenser toperiodicallyrapidly accumulate a charge such as to bias said gridbeyound cutoif whereby its period 0! discharge through said resistancesubstantially determines the frequency of the oscillations.

- 5. In a device for the production oi musical sound, an oscillatorcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a plate-cathode circuit and a grid-cathode circuit, aresistance connected in said grid-cathode circuit, ieedback means havingtwo closely coupled portions one of which is connected in saidplatecathode circuit, a condenser connected at one side to said grid andat the other side to the other-of said portions, said condenser being inelectrical connection through said other portion with said cathode. mans for supplying operating potentials to said plate and grid. saidfeed-back tially determines the frequency or the oscillations, and meansfor adjusting the grid-operating potential to effect frequencyadjustment.

6. In a device for the production of musical sound, an oscillatorcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a plate-cathode circuit and a grid-cathode circuit, resistancemeans connected in said grid-cathode circuit, feed-back means having twoclosely coupled portions one of which is connected in said plate-cathodecircuit, capacity means connected at one side to said grid and at theother side to the other of said portions, said capacity means being inelectrical connection through said other portion with said cathode, andpotentialsuilplying means for operating said tube, said feed-back meansbeing poled to cause said capacity means to periodically rapidlyaccumulate a charge such as to bias said grid beyond cut-oil! wherebyits period of discharge through said resistance means substantiallydetermines the frequency of the oscillations, at least one of saidresistance and capacity means being variable to effect frequencyvariation.

"If In a'device for the production of musical sound, a thermionic tubehaving a plate, a grid, and a heated cathode, a plate-cathode circuitand a grid-cathode circuit, a resistance and a switch connected inseries with one another in said grid-cathode circuit, feed-back meanshaving two closely coupled portions one of which is connected in saidplate-cathode circuit, a condenser connected at one side to said gridand at the other side to the other of said portions, said condenserbeing in electrical connection through said other portion with saidcathode, potential-supplying means for operating said tube, saidfeed-back means being poled so that upon closing of said switch saidcondenser is caused to periodically rapidly accumulate a charge such asto bias said grid beyond cutoff whereby its period of discharge throughsaid resistance substantialiy determines the frequency of theoscillations, and means for normally supplying cutoil' potential to saidgrid.

.8. In a device for the production of musical sound, a thermionic tubehaving a plate, a grid, and a heated cathode, a plate-cathode circuitand a grid-cathode circuit, a plurality of resistances, a plurality ofoperating switches respectively connecting said resistances in saidgridcathode circuit, feed-back means having two closely coupled portionsone of which is connected in, said plate-cathode circuit, a condenserconnected at one side to said grid and at the other side to the other ofsaid portions, said condenser being in electrical connection throughsaid other portion with said cathode, and potentialsuppiying means foroperating said tube, said feed-back means being poled so that uponactuation of any of said switches said condenser is caused toperiodically rapidly accumulate a charge such as to bias said gridbeyond cutoff whereby its period of discharge through any of saidresistances substantially determines the frequency of the oscillations,said resistances being of difl'erent values to effect the establishmentof different frequencies.

9. In a device for the production of musical sound, a thermionic tubehaving a plate, a grid, and a heated cathode, a plate-cathode circuitand a grid-cathode circuit, a plurality of resistances, a plurality ofswitches respectively connecting said resistances in said grid-cathodecircuit, feedback means having two closely coupled portions one of whichis connected in said plate-cathode circuit, a condenser connected at oneside to said grid and at the other side to the other of said portions,said condenser being in electrical connection through said other portionwith said so,ss1

cathode, and means for supplying operating potentials to said plate andgrid, said feed-back means being poled so that upon actuation of any ofsaid switches said condenser is caused to pcriodically rapidlyaccumulate a charge such as v to bias said grid beyond cutoff wherebyits period of discharge through any of said-resistances substantiallydetermines the frequency of the oscillations, said resistances being ofdifferent values to eflect the establishment of different frequencies,said potential-supplying means including a plurality of tuningpotentiometers respectively connected in said grid-cathode circuit bysaid switches.

10. In a device for the production of musical sound, two oscillatorseach comprising a thermionic tube having a plate, a grid, and a heatedcathode, a plate-cathode circuit and a gridcathode circuit, a resistanceconnected in said grid-cathode circuit, feed-back means having twoclosely coupled portions one of which is connected in said plate-cathodecircuit, and a condenser connected at one side to said grid and at theother side to the other of said portions, said condenser being inelectrical connection through said other portion with said cathode,potentialsupplying means for operating both tubes, in each of saidoscillators said feed-back means being poled to cause said condenser toperiodically rapidly accumulate a charge such as to bias said gridbeyond cutofl whereby its period of discharge through said resistancesubstantially determines the frequency of the oscillations, saidoscillators being adapted to produce musically related frequencies, andcoupling means between said oscillators communicating attenuatedimpulses from one to the grid-cathode circuit of the other to cause saidone to control the irequency of said other.

11. In a device for the production of musicalsound, two oscillators eachcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a plate-cathode circuit and a grid-cathode circuit, aresistance connected in said grid cathode circuit, feed-back meanshaving two closely coupled portions one of which is connected in saidplate cathode circuit, and a condenser connected at one side to saidgrid and at the other side to the other of said portions, said condenserbeing in electrical connection through said other portion with saidcathode, potential-supplying means for operating both tubes, in each ofsaid oscillators said feed-back means being poled to cause saidcondenser to periodically rapidly accumulate a charge such as to biassaid grid beyond cutoff whereby its period of discharge through saidresistance substantially determines the frequency of the oscillations,said oscillators being adapted to produce musically related frequencies,and coupling means between said oscillators communicating attenuatedimpulses from one to the grid-cathode circuit of the other to cause saidone to control the frequency of said other, said coupling meansisolating said oscillators to prevent reverse control.

12. In a device for the production of musical sound, an oscillatorcomprising a thermionic tube having a plate, a grid, and a heatedcathode, a plate-cathode circuit and a grid-cathode circuit, aresistance connected in said grid-cathode circuit, feed-back meanshaving two closely coupled portions one of which is connected in saidplatecathode circuit, a condenser connected at one side to said grid andat the other side to the other of said portions, said condenser being ineiectri- 7 cal connection through said other portion with said cathode,and potential-supplying means for,

operating said tube. said feed-back means being poled to cause saidcondenser to periodically rapidly accumulate a charge such as to biassaid grid beyond cutoff whereby its period of discharge through saidresistance substantially determines the frequency of the oscillations,and a repeater tube circuit through which the oscillations are suppliedcomprising a second condenser and a second resistance connected inparallel with one another between the grid and cathode of the repeatertube, means for normally supplying cutoil potential to the grid of therepeater tube, a key, means cooperating with said key for momentarilysupplying operating potential to the grid of the repeater tube andsimultaneously therewith momentarily charging said second condenser, andmeans rendered operative by release of said key for normallyshort-clrcuiting said second condenser.

13. In an electrical musical instrument for the simultaneous playing ofa plurality of musical notes, the combination of a generator ofelectrical impulses of relatively high frequency, a first frequencydivider stage for producing electrical impulses of a lower harmonicallyrelated frequency, said first stage comprising a multi-electrodeelectron discharge device having an input circuit and two outputcircuits, the input circuit of said first stage including a firstconnection to supply a signal from said generator to said first stage, asecond frequency divider stage for producing electrical impulses ofstill lower harmonically related frequency, said second frequencydivider stage comprising an electron discharge device having an inputcircuit and an output circuit, one of the output circuits of said firststage including a second connection for carrying a signal from saidfirst frequency divider stage to the input circuit of said second stage,a common transmission circuit, the other output circuit of said firststage including a third connection for carrying a signal from said firststage to said common traon circuit, the output circuit of said secondstage including a fourth connection for carrying a signal from saidsecond stage to said common tran circuit, an impedance in one of theoutput circuits of said first stage for preventing undesirable reactionbetween the two output circuits of said first stage, a key to complete acircuit through said which said impedance is in said first mentionedoutput circuit of said first stage, said impedance consisting of theimpedance between an additional grid and other electrodes included inthe electron discharge device of said second frequency divider stage.

16. The combination set forth in claim 13 in which said impedance is insaid second mentioned output circuit of said first frequency dividerstage,

said impedance consisting of the grid to other electrode impedance ofthe input circuit of an additional key-controlled repeater tube.

17. The combination set forth in claim 13 in which the manner ofoperation of one of said keys determines the intensity envelope of thesignal carried to said common transmission circuit.

18. The combination set forth in claim 13 in which said impedance ispartially controlled by the operation of one of said keys.

19. The combination set forth in claim 13 in which each of saidfrequency divider stages comprises a multi-element discharge device ofthe vacuum type.

20. The combination set forth in claim 13 in which the electrondischarge device in said second stage is a vacuum tube having aplurality of grids effectively placed to modulate the electron stream insaid device.

21. The combination set forth in claim 13 in which there is an impedancein each of the output circuits of said first frequency divider stage forpreventing undesirable reaction between said output circuits of saidstage.

22. The combination set forth in claim 13 in which an electron dischargedevice, acting as a switch. effects the completion of the circuit.

23. The combination set forth in claim 13 in which an electron dischargedevice, acting as an impedance varying with time, controls theefi'ectiveneu with time of one of said circuits completed by theoperation of the key.

GILBERT

